Photovoltaic devices, commonly known as solar cells, are known in the art for converting the sun's solar energy into useful electrical energy. These solar cells typically comprise a matrix or array of semiconductor spheres embedded in a light-reflective aluminum foil, the semiconductor material typically comprising silicon. One such solar cell is disclosed in U.S. Pat. No. 4,021,323 to Kilby et al., incorporated herein by reference, which includes a solar array composed of a transparent matrix provided with particles of silicon, each particle having a P-region exposed on one matrix surface, and an N-type region extending to an opposed matrix surface. Electrical energy is produced when photons of light strike the silicon sphere and induce electrons to cross the depletion region between the two conductivity types. Another solar cell is disclosed in U.S. Pat. No. 5,028,546 to Hotchikss, incorporated herein by reference.
One previous method of fabricating silicon spheres involves shotting molten purified silicon out of a nozzle, or out of a rotating disk. The spheres formed in this manner are highly irregular in shape and are polycrystalline. These spheres can later be made crystalline with the use of another process, involving re-heating the material above the melting point, and then controllably cooling the material.
Another process for producing crystalline silicon spheres is disclosed in U.S. Pat. No. 4,637,855, incorporated herein by reference. Silicon spheres are fabricated by applying a slurry of metallurgical grade silicon onto the surface of a substrate capable of maintaining integrity beyond the melting of silicon. The layer of silicon slurry is then patterned to provide regions of metallurgical grade silicon. The substrate and silicon slurry are then heated above the melting point of silicon. The silicon beads from the slurry to the surface as relatively pure silicon, and forms silicon spheres due to the high cohesion of silicon. The spheres are then cooled below the melting point of silicon, and the silicon spheres then crystalize.
Other processes, such as disclosed in U.S. Pat. No. 5,069,740 to Levine et al. and incorporated herein by reference, include using a sieve to separate irregular metallurgical grade silicon particles by size. Particles are obtained within a desired size range, and then repeatedly melted and cooled to draw impurities from the silicon to a silicon dioxide skin formed during each melting process. In this manner, particles having a desired degree of silicon purity can be obtained. This silicon dioxide skin is removed after each melting and cooling cycle.
To realize solar cells of high efficiency, it is necessary that the semiconductor spheres be of high purity. High purity silicon spheres can ultimately be obtained by starting with either metallurgical grade, or semiconductor grade silicon. However, the greater the impurity of the starting material, the greater number of melting and grinding cycles that will be necessary to ultimately obtain high purity semiconductor spheres. This purification process is time consuming, requires a substantial amount of energy, and is expensive. These melting cycle considerations need to be balanced against the cost of the starting material. In the past, the cost of semiconductor grade silicon feedstock was very expensive in relation to metallurgical grade silicon feedstock. More recently, however, the cost of off-spec semiconductor grade silicon feedstock is more in line with the cost of metallurgical grade silicon feedstock. Obviously, it is desirable to start with semiconductor material of higher purity where cost and availability parameters permit.
To obtain high efficiency solar cells, not only should the silicon spheres be of high purity, they should also be of uniform size and mass. A typical silicon sphere diameter is preferred to be about 30 mils. This size requirement can be difficult to obtain in high yields. Usually, the more melt cycles that are required and implemented, the lower the ultimate yield of a run.